Try writing the book first, without worrying about the music yet. Sit down, and think about what kind of a story you'd like to tell, and who you'd like to tell it with. (Hint: the smaller the cast, the easier it is to stage.) For your first attempt at writing, you're probably going to want to write a one-act play. The best one acts take place in one location, so modify your idea of your story to take place in one location. Like any story, it needs a beginning, middle, and end. Once you have an idea of the story you want to tell, write it down in its simples form - summarise the plot on a piece of paper, no more than a page or so. The plot can be in the '20s or in ancient Egypt...it can also be in the sandwich shop down the street. It doesn't have to be exotic to be fun. (Hint: You're better off writing what you're familiar with, especially at first.)
Once you know the plot, write the book - that is, the non-song part of the play. When you find something that would make a good song, just jot a little note for yourself, saying what information the song will contain, and keep going with dialogue after it ends. The point here isn't to have a final draft of the book, it's to get to know the story and the characters - it's easier to meet them in prose than in rhymed lines at first.
Once you have this done, go through and write the songs. With each song, think about what information you're trying to convey, and make sure that it comes across. Focus on making it sound like they're talking and NOT just making it cheesy. (Sondheim is my personal deity for all things musical theatre, but you'll have to find people you like to inspire you.)
After the songs are written, revisit the book and make sure it all flows seamlessly. Then, when you're ready, work on the music!
Once it's all finished, get some friends together for a staged reading to make sure it sounds as good in reality as it does in your head. Good luck!